The present invention relates to an artificial mensicotibial joint for a knee joint prosthesis of the kind which includes at least one tibial component on which the meniscus is movably disposed.
Different ways of dealing with knee joint destruction and other joint diseases or malformations in the knee joint through prosthetic surgical intervention are already known in the art. Disease-caused defects which may be concerned are rheumatic knee joint diseases and wear injuries which affect in the first instance joint cartilage and in the second instance underlying bone tissue which is worn away so that a varying degree of joint defect occurs.
The normal knee joint is based on a three-component relationship which permits both freedom of movement and firmness as well as a large contact area whereby concentration of stress is avoided. The two main components in the knee joint are the femur (thigh bone) and tibia (shin bone), the articulatory surfaces of which are connected to each other through a ligamentary apparatus in the form of ligaments and cruciate ligaments. The third component, the meniscus, is localized between the articulatory surfaces of the femur and tibia and functions as a movable shock-absorber. The meniscus accompanies the femur during rotation and the tibia in bending and stretching. The joint between the meniscus and femur may be designated as constrained, which means that there is good congruence between the articulatory surface of the femur and that of the meniscus (jont ball-joint socket relationship). The joint between the meniscus and tibia, in contrast, on account of the mobility of the meniscus and its relation to the tibia, is designated as non-constrained, permitting the tri-axial mobility which is necessary for, among other things, normal walking.
Several different types of prosthesis are already known in the art, both types of prosthesis which comprise only a femoral and tibial component and types of prosthesis which also include a third component which may be either fixed or movable (meniscus). With regard to different types of prosthesis and the requirements imposed on these as regards stability, resistance to mechanical wear, flexibility, etc., reference should be made to our parallel patent application Ser. No. 801,705.
There are principally two lines of designs of mobile tibial bearing elements available today, the non-constrained Oxford type as discussed in U.S. Pat. No. 4,085,466 to Goodfellow et al and the constrained New Jersey type of artificial meniscus as described in U.S. Pat. No. 4,309,778 to Buechel et al. The Oxford prosthesis, which is intended for cement fixation and consists of separate components for each joint chamber. It comprises an anatomically cupped femoral condylar prosthesis of metal and a metal tibial prosthesis, the upper surface of which is flat and articulates against the flat, lower surface of the "meniscus". This is a polyethylene disk located between the femur and the tibia. The upper surface of the meniscus facing towards the articulatory surface of the femoral prosthesis is cupped so that it fits well against the convexity of the femoral prosthesis. This contact may be said to be constrained. At the same time, the joint between the tibia and the lower surface of the meniscus is non-constrained, which means that the meniscus is self-locating with respect to the tibia. This combination of a constrained and a non-constrained joint affords the advantages of both these designs, a high degree of congruence and freedom for sliding movements in the horizontal plane.
In the Oxford prosthesis the menisco-tibial joint is stabilized only by the surrounding joint capsule and ligaments. In several cases, the meniscus has been reported to dislocate. For the most part it has then been squeezed posteriorly during knee flexion, probably because there are separate pivot points for the medial collateral ligament and the spherical femoral prosthetic surface respectively.
The femoral articulatory surface of the New Jersey prosthesis has a non-spherical femoral surface which offers less contact area towards the meniscus than that of the Oxford knee. The menisco-tibial joint is constrained by a steering strip along the lower surface of the meniscus, running in a curved dovetailed track in the tibial prosthesis. The track emerges towards the front and rear against the capsule enclosure giving sufficient firmness to prevent the meniscus from moving out of the joint. Since the track runs forwards and rearwards and describes a portion of the periphery in a circle with its center towards the center of the knee joint, the artificial meniscus will permit a certain rotation between the femoral and tibial prosthesis both of which are of metal and are intended for cement-free fixation in accordance with the porous coating principle. The knee prosthesis of Polyzoides et al (the gliding meniscus knee--Zimmer) is with respect to the meniscus more or less identical to the New Jersey knee.
The pros of the reduced risk of dislocation have to be weighed against the cons of the constraint. The New Jersey knee design seems to be very sensitive to surgical malalignment due to the complexity of the kinematics of the prosthesis. Disturbance of the delicate synchronization between, on the one hand, the tension of the ligaments and, on the other the three-dimensional arrangement of the four articulating femuro-tibial joint surfaces, could easily lead to reduction and even locking of menisco-tibial movements. Then the whole idea of the meniscus collapses. This is also the case if the components of the prosthesis are perfectly well aligned but fibrous tissue grows into the track and reduces the movements of the meniscus: This risk is obvious. Moreover, the slideways of the medial and the lateral meniscus do not have the same pivot point, which means that the meniscuses are likely to lock each other in certain positions where the bearing surfaces are not in harmony with the cruciates. Nor does the design make allowance for a relationship which applies to the natural knee, namely that the scope of movement of the meniscus in relation to the tibia is greater on the lateral side than on the medial side, which presumably contributes to inward rotation of the femur on the tibia at the final stage of knee extension. This rotational movement is believed to benefit stabilization of the knee via the so-called screw-home mechanism.